The present invention relates to an amorphous silicon solar cell used in a solar cell power generation system.
The arrangement shown in FIG. 3 U.S. Pat. No. 2,530,408 is known as a conventional solar cell.
A transparent electrode 2, a power-generating film 3, and a metal electrode 4 are formed in this order on a glass substrate (or transparent film) 1. The power-generating film 3 is constructed of a p-type a-Si.sub.x C.sub.1-x :H layer (p layer) 5, an i-type a-SiH layer (i layer) 6, and an n-type a-Si:H layer (n layer) 7. The p layer 5 as a window layer is made of amorphous silicon carbide to increase its optical forbidden band width. Also, to control the valence electron of the p layer 5, diborane (B.sub.2 H.sub.6) is added as a doping gas to the source gas in the formation of the p layer 5. In the solar cell with this construction, light enters from the glass substrate 1, passes through the transparent electrode 2 and the p layer 5, and reaches the i layer 6 where the optical energy is converted into electrical energy.
Since the i layer 6 is a light-absorbing/power-generating layer in the solar cell shown in FIG. 3, the film properties of this i layer 6 have a direct influence on the characteristics of the solar cell. Also, the output current of the solar cell is determined by the light absorption characteristic and film thickness of the solar cell material. That is, if the absorption coefficient is small, the film thickness must be increased; if the absorption coefficient is large, the film thickness can be small. In the amorphous silicon solar cell, the absorption coefficient to light of 500 to 700 nm having the greatest influence on the output current increases as the number of defects in the amorphous silicon film forming the i layer 6 decreases, i.e., as the number of unbonded hands of Si decreases. For example, when a film having a defect density of 3.times.10.sup.15 defects/cc is applied to the i layer of a single type amorphous silicon solar cell, the film thickness of the i layer 6 must be 400 nm or more to increase the initial efficiency as shown in FIG. 4. However, if the film thickness of the i layer 6 is thus increased, a long time is required to form this i layer 6, lowering the productivity.
Additionally, if the film thickness of the i layer 6 is increased to raise the initial efficiency, the stabilization efficiency declines because the rate of optical degradation increases. On the other hand, if the film thickness of the i layer 6 is decreased, the degradation rate decreases. However, as shown in FIG. 5, the stabilization efficiency does not increase because the initial efficiency is small.